Background . Several Canadian provinces are extending the interval between COVID-19 vaccine doses to increase population vaccine coverage more rapidly . However, immunogenicity of these vaccines after one dose is incompletely characterized, particularly among the elderly, who are at greatest risk of severe COVID-19 . Methods . We assessed SARS-CoV-2 humoral responses pre-vaccine and one month following the first dose of BNT162b2 mRNA vaccine, in 12 COVID-19 seronegative residents of long-term care facilities (median age , 82 years), 18 seronegative healthcare workers (HCW; median age , 36 years) and 4 convalescent HCW . Total antibody responses to SARS-CoV-2 nucleocapsid (N) and spike protein receptor binding domain (S/RBD) were assessed using commercial immunoassays . We quantified IgG and IgM responses to S/RBD and determined the ability of antibodies to block S/RBD binding to ACE2 receptor using ELISA . Neutralizing antibody activity was also assessed using pseudovirus and live SARS-CoV-2 . Results . After one vaccine dose, binding antibodies against S/RBD were ~4-fold lower in residents compared to HCW (p <0.001). Inhibition of ACE2 binding was 3-fold lower in residents compared to HCW (p=0.01) and pseudovirus neutralizing activity was 2-fold lower (p=0.003). While six (33 %) seronegative HCW neutralized live SARS-CoV-2, only one (8 %) resident did (p=0.19). In contrast, convalescent HCW displayed 7- to 20-fold higher levels of binding antibodies and substantial ability to neutralize live virus after one dose . Interpretation . Extending the interval between COVID-19 vaccine doses may pose a risk to the elderly due to lower vaccine immunogenicity in this group . We recommend that second doses not be delayed in elderly individuals.